The invention relates to an artificial knee joint for a knee, in which the rear cruciate ligament and collateral ligaments are operative, consisting of a metal platform, which on its under side is connected via attachment members to the tibia, and consisting of metal femur condyles and one or several slide members, which transmit forces between the metal platform and the metal femur condyles.
The problems of and solutions for artificial knee joints are described in detail in U.S. Pat. No. 4,309,778. The solutions shown are supposed to imitate the movement mechanism of the natural knee joint and to make its design as safe as possible with respect to the guidance of the movable components. They require a corresponding large amount of space for safeguarding the guiding movement, around which a resection in the osseous tissue of the tibia bone has to be performed.
In the natural intact knee joint as a simplification three components of the relative movements between the femur and the tibia condyles can be identified:
Translational anterior-posterior movement (sliding) PA1 Rolling of the femur condyles on the tibia condyles when the knee is flexed PA1 Rotation of the tibia with respect to the femur around an axis parallel to the tibia axis.
To what extent these movements are possible after the implantation of a sliding prosthesis depends on the respective construction and condition of the ligaments and muscles. Prostheses subject to less constraint permit greater translational and rotatory movements of the tibia relative to the femur. Such "less constraint" prostheses are characterised by a slighter congruence of the femur and tibia condyles and a smaller intercondylar eminentia which restricts movement less.
With the construction having two separate tibial sliding members made from polyethylene described in U.S. Pat. No. 4,309,778, translational and rotatory movements are produced in the plane of the joint by the displacement of the tibial slide components guided in the groove-shaped rails. These guide rails are disposed on tracks having the shape of a circular segment. The sliding movement of the femur condyles on the tibia condyles required for knee flexion is performed between surfaces which are largely congruent to one another, with the supporting region decreasing as the rotation around the tibia axis and/or the translation at right angles to the tibia axis increase. However reduced congruence of the sliding surfaces is associated with greater surface pressure, may result in local overstressing, an increase in polyethylene abrasion and the rapid deterioration of the tibia condyles made of polyethylene. The "menisci" guided separately in the rails have slight lateral clearance so that they can be centered by this restricted free mobility with respect to the femur condyles. The movement of the polyethylene components in the grooves may be impeded by deposits, tissue cells and by polyethylene abrasion.